Potentiometer and contact therefor



July 2, 1957 s. B. RASMUSSEN 2,798,137

POTENTIOMETER AND CONTACT THEREFOR Filed Aug. 51, 1955 /N VENTO/P.Suzi/v B. Rnsmussalv BY Hi5 HTTORNQ'YS. V HARE/s, K/scrg. Fosrsle &Hake/.5

United States Patent G POTENTIOMETER AND CONTACT THEREFOR Svein B.Rasmussen, South Pasadena, Calif., assignor, by

mesne assignments, to Beekman Instruments, Inc., Fullerton, Calif, acorporation of California Application August 31, 1953, Serial No.377,535

9 Claims. (Cl. 20148) have been used without a complete study oranalytical understanding of factors which must be considered in thedesign of a fully satisfactory precision-type potentiometer.

Two important problems which arise are the noise voltages created by thecontact as it passes over the potentiometer windings and the wear of thecontact and of the windings. Noise has been found to result from poorcontact or from a transiently high resistance or open circuit as thecontact moves across the windings. It is often amplified by othercircuit elements and can sometimes lead to harmful results, such asupsetting electrical computer circuits which are coupled with high gainamplifiers.

As a moving potentiometer contact approaches an in dividual turn of awire-wound resistance element, the contact has a certain amount ofkinetic energy which may be computed by the formula E= /2 mv where E isthe kinetic energy, in is the mass of the contact, and v is its linearvelocity in a forward direction transverse to the turns of the winding.lish and maintain contact with the turns, it is designed to bear againstthe windings with a biasing force which may, under static conditions, beexpressed simply by the constant F. When the contact strikes a turn ofthe resistance wire, it is displaced minutely in a direction opposite tothe biasing force and a certain fraction K of the kinetic energy of thecontact must be absorbed by the spring or resilient means which holdsthe contact against the winding. This absorbed energy results in adeflection of the contact away from the winding, and

the deflection due to a single contact impulse has been found byanalysis to be represented by the formula where x equals the deflectionand the remaining symbols are as have been defined. It has been foundthat these minute deflections are a major cause of noise in wirewoundotentiometers.

The deflection x represents an actual movement of the contact away fromthe winding. Generally, a completely closed contact represents aresistance of 1 ohm or less, while an air space in excess of 1 milrepresents an infinite resistance at ordinary voltages. The deflection xis, however, customarily of the order of .1 mil or less. The exactamount of resistance which is introduced by the deflection x isdifficult to define due to limited presentday knowledge regarding theelectrical phenomena associated with contact openings of this order ofmagnitude.

In order for the contact to estab- His a well-known fact, however, thatsuch deflections do greatly increase the resistance and result in avariation in contact voltage.

Referring to the formula for computing the deflection x, it may be seenthat the amount of the deflection may be'reduced by increasing the forceF with which the contact bears against the winding. This results,however, in increased wear of both the contact and the windings, with aresulting shorter life for the potentiometer, not to mention increasedoperating torque. Decreasing the linear velocity of the contact isanother possibility, but this is usually determined by the user of thepotentiometer and is beyond the control of the manufacturer.

The factor K, which may also be considered a measure of roughness, or aroughness factor, could bemade quite small by lapping the contact andresistance element together, as with a polishing compound. Still, thereis a practical limit to how smooth a resistance element can be made.Moreover, the smoothing operation, by thinning the windings, wouldshorten the potentiometer life, and also seriously and adversely affectlinearity and precision of the resistance value.

This leaves the possibility of reducing the mass of the potentiometercontact in order to reduce the deflection. The problem is one ofproviding a contact which is light in mass, but which will neverthelessprovide optimum contact when used in connection with a potentiometerresistance element which is convex in shape. Prior workers in this arthave not recognized the critical relationship between contact mass andnoise, nor the possibility of solving the very pressing noise problem byproper contact design.

It is an object of the invention to provide a contact, for use in aresistance means, which develops a minimum amount of noise voltage andhas a maximum useful life. With my invention, reductions in noisevoltage of are common and reductions from several volts to less than amillivolt have sometimes been achieved. Wear measurements indicate thatthe amount, in volume, of my contact which is worn off is only 5-20% ofthe amount worn off standard contacts under similar conditions.

In essence, my invention contemplates a contact, for use in a resistancemeans, which is light in mass, which is preferably formed of wire orother suitable material, and which is shaped to provide at least twostable points of contact with the winding on opposite sides of a planepassing through the resistance element, the contactmaking elements beingstabilized and restrained from any tendency to separate from each otherin consequence of their contact with oppositely-curved or slopingsurfaces of the convex resistance element.

It is a further object of my invention to provide a contact which,because of outstanding low wear characteristics, may be used with alltypes of resistance wire, including those made of soft copper-nickel andmagnesium alloys. This obviates the necessity for stocking several typesof contact materials for use with different types of resistance wire, asis customary at the present time.

Another object is to provide a contact of small overall dimensions whichmay be used in applications where space is limited. A further object isto providea contact on which, effectively, any desired contact faceradius can conveniently be obtained, as demanded by varying diameters ofresistance wire and spacing of turns.

It is an object of my invention to provide a contact which, because ofits smaller mass, will provide better performance under conditions ofvibration and shock than contacts which are presently in use. My contactwill also remain noise-free at considerably higher operatresonantfrequency.

It is an object of my invention to provide a potenti-- ometer andcontact therefor having twocontact surfaces instead of one, so thatnoise voltages are reduced, particularly where the potentiometer isexposed to contamination from dirt, oxides, etc. Another objectis-toprovide a contact having a smoother motion and less wear by beingoperated along its own grain directioninsteadof against the grain, as iscommonly the case at-the'present time.

It is an object of the invention to provide potentiometers and contactstherefor having oneor moreofthe features or advantages discussedabove.

While I haveshown in the accompanying drawings and detailed descriptionpreferred embodiments of my invention, it should be understood that thesame are susceptible of modification and change Withoutdeparting fromthe spirit of my invention.

In the accompanying drawing:

Fig. 1 is an elevational view, with the cover removed, ofa potentiometerequipped with oneembodiment of contact means of the invention; 3 1

Fig. 2 is a longitudinal sectional view taken along the line 22 of Fig.1; t

Fig. 3 is an enlarged elevationalview of the contact means of Fig. 1;

Fig. 4 is a sectional view taken along the line 44 of Fig. 3;

Fig. 5 is a perspective view of the contact means of Figs. 1, 2, 3 and4; and

ig. 6 is a perspective view of an alternativeembodiment of the contactmeans of the invention.

The invention may be used with straight, single-turn ormulti-turnpotentiometers. A single-turn embodiment which has been selected toillustrate my invention comprises a potentiometer 10 having an outerhousing 11. Mounted Within the housing 11 is a circular resistanceelement 12, which comprises a core 13 which is formed from a circularturn of wire, around which is wound a large number of turns ofrelatively fine-gauge resistance wire, comprising a resistancewinding-14.- The turnsare space-wound or otherwise electricallyinsulated fromeach other.

Extending through the axis of housing 1'1 and rotatably mounted thereinis a shaft 15. A rotor 16 is attached to shaft-15 and adaptedto-berotatedthereby. Rotor- 16 is composed of electricallynon-conducting; material.

Mounted on one side of rotor 16' is a circular slip r-ing 17' which iselectrically conductive.

Extending from the outside of housing II are three electrical terminals18, 19 and 20. Terminals=19-and 20 are respectively connected totheopposite ends of A brush 21 is attachedto the to bring contact points23 intoresilient contact with theouter surface of slip ring 17. Thecontact. points 23 of brush 21 travel-alonga pathadjacent the inner circumference of slip ring 17 whenshaft15 is rotated with respect tohousing 11'.

Means for establishing an electrical contact between slip ring 17 andresistance winding 14 is provided by a contact 2.4, which ismounted onthe outside. of rotor 16 on a base 25, which is attached andelectrically connected to slip ring 17. Base 25 is a contact base and isbent at a right angle adjacent the edge of rotor 16 to extend along theoutside thereof.

Attached to the bent portionof base 25, as by welding or soldering, arethe base or rear portions of the con: tact 24. This contact is shown ascomprising a loop of wire, typically a wirewhich is bent back uponitself to form a pair of substantially parallel contact elements or arms29, and joined by a bridge 31adjacent theends thereof. While variousbridge meanscan. be used to from each other.

connect the arms, a section of the wire, arched as shown in Fig: 4,-is-preferred. Arms 29- and 30 are provided adjacent their joined endswith a pair of arcuate portions 32 and 33 which are held in resilientengagement with side portions of the resistance element by use of anysuitable biasing means, such as the arms 29 and 30, or either of them.The points of engagement occur on the surfaces of arcuate portions 32and 33.

In use, connections to external circuit elements are customarily made atterminals 18, 19 and 20 of the potentiometer 10. The resistance betweenterminals 19 and 20 is the total resistance of winding 14. The amount ofresistance between terminal 18 and terminal 19 or between terminal 18and terminal 20 is determined by the position of contact 24, which is inturn dependent upon the rotation of shaft 15.

Referring to Fig. 3 of the drawings, it should be noted that whileresistance element 12 is circular, any small portion thereof which isengaged by contact 24 can be considered as substantially cylindrical inshape. Referring to Fig. 4, it may be seen thatresistance element 12 hasa convex, exterior orexposed surface withside portions which areoppositely sloped on opposite sides of a longitudinal planepassing-through resistance element 12. The arcuate portions 32- and 33-of-arms 29' and 30 thus engage oppositely-sloping portions of resistanceelement 12,- which portions tend to force arms29'and 30apart Bridge 21prevents suchseparation by joining arms 29 and 30;together at theopposite ends of arcuate portions 32' and 33 to hold arms 29 and 30 infixed spaced relationship to each other. Without such bridgemeansjoiningarms 29-and 30', the movement of contact 24' along resistance element 12would force arms 29 and 30 apart, resulting in poor electrical contactand increased noise voltage.

Another embodiment of my invention is shown in-Fig. 6 'of the drawings.Insuch embodiment, the-contact com- 'prisesa single arm 34, formedfromapiece of wire which isbent back upon itself-in the shape of a loop.The loop is provided with a pair of arcuateportions 35 and 36-, whichare joined by a bridge-37w In this embodiment of my invention, bridge 37serves the same purpose of preventing the portions of the contactwhichengage oppositely-sloping side portions of resistance element 12from being separated from each other.

If desired, the arm or arms of the contact and the biasing means may beseparate, although it is, desirable that the arm or armsbe somewhatresilient to aidin biasing the contact into engagement with resistancewinding 12.

In operation, my contacts provide less noise voltage and less wear. Onereason;for the superior performance of my contacts is that they havegreater agility in following the contours of successive winding turns,of the resistance element, i. e., in mountingand dismounting theasperities which this discontinuous structure represents, while previouscontacts have tended to dig into the asperities, producing longitudinalscore marks and a condition of sustained roughness. Another benefit ofmy contacts is that the sliding motion is smoother along the grain ofthe contact rather than across the grain, resulting in less noisevoltage and less wear due to such smoother movement.

Extensive and accurately-made-tests, comparing potentiometersemployingcontacts of'the invention with potentiometers equipped with conventionalpresent-day contacts show that the invention is capable of reducing'wearmanyfoldand to such an extent that wear is practically negligible evenafter, 1,000,0(l0-revolutions of the rotor; also thatnoise is verymarkedly reduced, sometimes as much as or. more, both initially andafter suchnumber of revolutions.

Various changesand modifications can, be made withoutdeparting from thespirit of the invention as defined in the. appended claims.

I claim as my invention:

1. A resistance means having a resistance element formed withoppositely-sloping portions on opposite sides of a longitudinal plane ofsuch resistance element, a contact means, and means for guiding saidcontact means for movement relative to said resistance element in adirection parallel to said longitudinal plane, said contact meansincluding a spring contact extending substantially in said directionparallel to said longitudinal plane and biased toward said resistanceelement, said contact having sideby-side spaced contact arms extendingsubstantially in said direction and lying on opposite sides of saidplane, said contact arms respectively engaging said oppositelyslopingportions of said resistance element in opposed contact areas as the soleelectrical connection between said spring contact and said resistanceelement during said relative movement, said contact including a bridgemeans integral with said contact arms and interconnecting same againstspreading under the bias of said spring contact when such contact armsengage said oppositely-sloping portions, said bridge means being on oneside of a transverse plane traversing said resistance element andpassing through said contact areas.

2. A combination as defined in claim 1 in which said means for guidingsaid contact means for relative movement in said direct-ion includes acontact base, said contact arms comprising a mounting portion on theopposite side of said transverse plane, said mounting portion beingsecured to said contact base.

3. A resistance means having a resistance element formed with alongitudinal convex contact zone, said contact zone havingoppositely-sloping side portions on opposite sides of a longitudinalplane passing through the center of said resistance element, a contactmeans relatively movable in a longitudinal direction with respect tosaid contact zone, means for biasing at least a portion of said contactmeans toward said contact zone of said resistance element, said contactmeans including a contact having spaced contact elements extending sideby side along said resistance element in said longitudinal direction andrespectively adjacent said oppositely-sloping side portions thereof,said contact elements having at least one mounting portion adjacent oneend thereof and having opposed contact surfaces adjacent the other endthereof, said contact surfaces respectively engaging theoppositelysloping side portions of said contact zone in opposed contactareas spaced from each other in a plane transverse to said resistanceelement, the engagement of said opposed contact surfaces with saidopposed contact areas being the sole electrical connection between saidcontact means and said resistance element during such relative movement,the pressure exerted by said biasing means tending to move said contactelements farther apart, and bridge means extending between said otherends of said contact elements to maintain same in substantially fixedspaced relationship to each other irrespective of the pressure exertedby said biasing means, said bridge means being disposed near the contactsurfaces of said contact elements.

4. A combination as defined in claim 3 in which said bridge means isdisposed entirely on one side of said plane transverse to saidresistance element.

5. A combination as defined in claim 3 in which at least one of saidcontact elements has a resilient portion integral therewith, saidresilient portion holding the contact surface of such contact element incontinuous engagement with said resistance element during relativemovement thereof.

6. A resistance means comprising: a resistance element comprising asubstantially cylindrical core and a plurality of turns of resistancewire wound helically on said core, said turns compositely providing aconvex surface hav ing oppositely-sloping portions on opposite sides ofa llongitudinal plane passing through the center of said core; a contactrelatively movable longitudinally of said resistance element andtransversely with respect to said turns, said contact providing twocontact arms extending side by side substantially parallel to but onopposite sides of said plane and being spaced from each other laterallyof the direction of relative movement, said contact arms providingcontact portions respectively engaging said oppositely-sloping portionsof said resistance element in opposed contact areas, such engagementbeing the sole electrical connection between said contact and saidresistance element during such relative movement, said contact includingbridge means interconnecting said contact arms and maintaining same insubstantially fixed space-d relationship to each other irrespective ofcontact pressure between said contact portions and saidoppositely-sloping portions, said bridge means extending between saidcontact arms near said contact portions thereof; and means for mountingsaid resistance element and said contact t or such relative movementwith said contact portions of said arms pressing against said opposedcontact areas to produce said contact pressure, said last-named mountingmeans including a contact base carrying said contact, and means forguiding said contact base to move said contact in said longitudinalplane with said contact arms onsaid opposite sides thereof.

7. In a resistance means having a longitudinal convex resistance elementformed with oppositely-sloping p'ortions on opposite sides of alongitudinal plane of said resistance element and a contact baserelatively movable parallel to such plane, a contact means relativelymovable along said resistance element and secured to said contact base,said contact means including: a single integral length of wire bent backupon itself comprising two contact elements joined by a bridge sectionof said wire, said contact elements lying on opposite sides of saidplane and respectively engaging said oppositely-sloping portions of saidresistance element during relative movement therealong, said bridgesection maintaining said contact elements in substantially fixed spacedrelationship to each other, said bridge section extending between saidcontact elements near said resistance element.

8. In a resistance means having a longitudinal convex resistance elementformed with "oppositely-sloping portions on opposite sides of alongitudinal plane of said resistance element and a contact baserelatively movable parallel to such a plane, a contact means relativelymovable along said resistance element and secured to said contact base,said contact means including: a pair of parallel lengths of a piece ofwire joined by an integral bridge extending between the ends thereof,each of said parallel lengths including a contact element, said contactelements lying on opposite sides of said plane and respectively engagingsaid oppositely-sloping portions of said resistance element duringrelative movement therealong, said bridge section being adjacent thepoints of engagement of said contact elements with said resistanceelement, and maintaining said contact elements in substantially fixedspaced relationship to each other, said bridge section extending betweensaid contact elements near said resistance element.

9. A contact means as defined in claim 8, in which said parallel lengthsof wire are provided adjacent the ends thereof with arcuate portionsadapted to engage said resistance element, said bridge sectioncomprising an arch extending between the ends of said arcuate portions.

References Cited in the file of this patent UNITED STATES PATENTS2,454,986 Beckman Nov. 30, 1948 2,486,931 Edwards et a1. Nov. 1, 19492,542,113 Bowitz Feb. 20, 1951

